The invention relates generally to steam turbines and more specifically to lower exhaust hoods for the steam turbines.
The outer shell of a steam turbine low-pressure section is generally called the exhaust hood. The primary function of an exhaust hood is to divert the steam from the last stage bucket of an inner shell to the condenser with minimal pressure loss. Usually the lower half of the exhaust hood supports an inner casing and acts as the supporting structure for the rotor. The upper exhaust hood is usually a cover to guide the steam to the lower half of the hood. The hood for large double flow low-pressure steam turbines are of substantial dimensions and weight and usually are assembled only in the field. In many steam turbines, the inner case of the steam turbine, for example a double flow down exhaust unit has an encompassing exhaust hood split vertically and extending along opposite sides and ends of the turbine. This large, box-like structure houses the entire low-pressure section of the turbine. The exhaust steam outlet from the turbine is generally conically-shaped and the steam exhaust is redirected from a generally axial extending flow direction to a flow direction 90 degrees relative to the axial flow direction. This 90-degree flow direction may be in any plane, downwardly, upwardly or transversely. Thus the prior exhaust hoods for steam turbines constitute a large rectilinear structure at the exit end of the conical section for turning and diffusing the steam flow at right angles.
The lower half of the exhaust hood, split vertically from the upper half, directs the exhaust flow of steam to a condenser usually located generally beneath the exhaust hood. The lower exhaust hood typically supports the inner casing of the turbine and the associated steam path parts such as diaphragms and the like. The lower exhaust hood is further loaded by an external pressure gradient between atmospheric pressure on the outside and near-vacuum conditions internally. The lower exhaust hood shell is generally of fabricated construction with carbon-steel plates. Typical sidewalls for the lower exhaust hood are flat and vertically oriented. To provide resistance to the inward deflection of the sidewalls under vacuum loading, the lower exhaust hood traditionally has included internal transverse and longitudinal plates and struts. These internal transverse and longitudinal plates and struts form a web, generally underneath the turbine casing and extending to the sidewalls. Vertical sidewalls result in a stagnant flow region underneath the inner casing. Flat walled hoods require flow plates. Flow plates are used to prevent the rapid expansion of the exhaust steam after passing through a horizontal joint restriction between the inner casing and the exhaust hood.
The internal hood stiffeners and flow plates are costly. Further, the thick-walled plate used for the sidewalls is also costly. Prior attempts to stiffen exhaust hoods have focused on different combinations of internal stiffeners (pipe struts, plates) and wall thicknesses.
FIG. 1 illustrates typical arrangements of a low-pressure turbine 100 with an exhaust hood. An exhaust hood 10 includes an upper exhaust hood 15 and a lower exhaust hood 20, mating at a horizontal joint 22. An inner casing 25 is supported at multiple supporting pads 30 on the lower exhaust hood 20. To distribute the load from these pads to a foundation (FIG. 2) for the low-pressure turbine, various supporting structures are present in the form of transverse plates 35, beams 37 and struts 40. These transverse plates 35 avoid the suction effect of the sidewalls 45 and end walls 50 and they distribute the load applied on the hood due to loads on inner casing 25. The lower exhaust hood 20 may further provide a support location 55 for shaft seals (not shown) and end bearings (not shown) for the turbine rotor (not shown). The lower exhaust hood may include a framework 70 including support ledge 75 that may rest on the external foundation (FIG. 2).
The sidewalls 45 and end walls 50 may be constructed of flat metal plates 60 (FIG. 1), joined at seams by welding or other known joining methods. Because of the similarity of construction and function, both sidewalls and end walls may hereafter be referred to as “sidewalls”. The foundation may be comprised of concrete with an opening, including vertical walls, and sized to accommodate the lower exhaust hood with its vertical sidewalls within.
FIG. 2 illustrates an axial view of a typical exhaust hood for a steam turbine illustrating flat sidewalls and a restricted steam flow path. The exhaust steam flow 65 in the upper exhaust hood 15 must pass by the horizontal joint restriction 80 between the hood 10 and the inner casing 25 before reaching a rectangular chute region 95 that conveys the steam downward to the condenser opening 85 at the bottom of the lower exhaust hood 20. The condenser opening 85 is much larger than the horizontal joint restriction 80, resulting in a stagnant zone 97 underneath the inner casing 25. To avoid uncontrolled expansion downstream of the horizontal joint restriction 80, flow plates 98 are added. To control deflections of the chute region 95 due to the inward-acting pressure gradient, the transverse support plates 35 provide internal stiffening.
The problem previously has been addressed by putting transverse and stiffening plates through out the hood. The methodology heretofore followed has been to make hood stiff enough by adding material so as to avoid excess deflection. The problem is that to control the side and end wall deflections of the hood, transverse and stiffeners are required inside of the hood. The existence of these transverse and struts increases the complexity of the hood, increases the weight of the hood and creates aero blockages resulting in aero performance losses.
Accordingly, it may be desirable to provide an alternate hood structure that reduces cost, complexity and improves flow distribution.